In a receiver of a wireless communication apparatus which adopts the direct conversion receiving scheme (such as a Multi Band OFDM (Orthogonal Frequency Division Multiplexing) UWB (Ultra Wide Band) system compliant with a WiMedia standard, used as a PHY (physical layer) in Wireless USB, or the like), in which a packet is transmitted or received while performing frequency hopping for each symbol and demodulation on a receiver side is started bay performing carrier sensing at the beginning of the packet, it is an important technical challenge to perform correlation detection (CCA: Clear Channel Assessment detection) at a preamble part at the beginning of the packet received with frequency hopping, and further, AGC (Auto Gain Control)/AFC (Auto Frequency Control)/synchronization establishment stably at high speed. The correlation detection involves an operation from carrier sensing to synchronization and start of hopping. As such a high-speed correlation detection technique, a description in Patent Document 1, for example, is referred to.
Further, as a factor of fixed degradation of reception at a time of direct conversion reception involving frequency hopping, a DC (direct current) offset that occurs in a received baseband signal by self-mixing may be pointed out. Unless the DC offset that occurs in the receiver baseband portion is appropriately handled to be suppressed, the received signal and the DC offset may be both saturated due to a high gain of an analog baseband portion.
When a DC offset level differs for each frequency hopping, a jump in the DC offset occurs immediately after the hopping due to a DC cut-off capacitance element Ccut for preventing an increase in a DC signal in a stage subsequent to the analog baseband portion. The DC cut-off capacitance element Ccut performs high-pass filtering of subcarriers that are more distant than a subcarrier in the most vicinity of a DC component, based on nulling of the DC signal at a time of OFDM.
When the time constant of a transient response of the DC offset jump exceeds a zero padding period+a guard interval (70.1 nsec), which are non-signal periods, ISI (Inter-Symbol Interference: interference between symbols) is caused, and the non-signal periods are interfered. As a result, multi-path delay wave processing using cyclic prefix (Cyclic Prefix) cannot be simply performed. A remarkable fixed degradation of reception (that is, fixed degradation in a reception characteristic) is caused. Further, the DC offset jump occurs in a transient-response manner. Thus, once the DC offset jump has occurred, it becomes difficult to improve or solve degradation in the reception characteristic. A solution to such a problem of the DC offset jump is disclosed in Patent Document 2, or 3, for example.
A further increase in speed and capacity such as in a wireless packet communication apparatus is desired in the future in a trend toward ubiquitous broadband wireless communications. Then, importance is attached to technical continuity capable of solving each of the problems and achieving a higher speed with high robustness. A MIMO (Multi Input Multi Output) space division multiplexing method plays a part in the technical continuity. As clear from a subsequent description the present invention proposes a configuration of a receiver that solves the problems described above and may also assume a MIMO configuration. With this arrangement, improvement in performance and added value are supplied, while avoiding the above-mentioned problems.
An analysis of related arts by the present invention will be given below.
Patent Document 1 discloses configurations shown in FIGS. 6A and 6B as a frequency hopping wireless communication apparatus and a carrier sensing device in which a carrier sensing determination accuracy is high and the time needed for carrier sensing is short in a wireless communication system involving frequency hopping and carrier sensing. FIG. 6A corresponds to a configuration of the frequency hopping wireless communication apparatus in FIG. 1 in Patent Document 1. FIG. 6B corresponds to a configuration of a carrier sensing unit in FIG. 2 in Patent Document 1. The configurations and operation overviews of the frequency hopping wireless communication apparatus and the carrier sensing unit are as follows.
An RF (Radio Frequency) signal output from an LNA (Low Noise Amplifier) 102 is branched. A carrier sensing unit 106 performs carrier sensing by energy detection and correlation detection of RF signals. The carrier sensing unit 106 includes a plurality of bandpass filters (RF BPFs) 201 that can respectively pass frequency-hopped RF signals. For an output of an RF BPF 201 for each hopping frequency, an energy detector (power detection unit) and an RF correlation detector (auto-correlation unit) are arranged. With this arrangement, carrier sensing detection outputs that have been temporarily distributed on a time axis by hopping are detected without alteration. Delay units 203 for respectively compensating for delays corresponding to a hopping pattern are inserted. Finally, detection values for respective hopping symbols, which have been combined on time, axis by those delays, are integrated and added by an adder unit 204. Fast carrier sensing is sought by increasing the correlation value in an early processing stage.
When fast carrier sensing involving frequency hopping is implemented, it is assumed to perform energy detection or auto-correlation detection at an RF frequency at the time of branching of an RF signal. Thus, RF detection for each frequency hopping is needed. An increase in the number of the bandpass filters (RF BPFs) 201 associated with the RF detection and an increase in the circuit size, an increase in the cost, and an increase in power consumption due to a detection circuit configuration for each RF signal are brought about.
Next, a configuration of Patent Document 2 that aims at removal of DC offset in a baseband portion in a receiver in a Multi Band OFDM UWB direct conversion system that performs frequency hopping will be shown in FIG. 7. FIG. 7A corresponds to the configuration of a Multi Band OFDM UWB receiver in FIG. 5 in Patent Document 2. FIG. 7B corresponds to FIG. 9 (which is a diagram explaining local signal self-mixing) in Patent Document 2. FIG. 7C corresponds to FIG. 2 (which is a graph showing a relationship between an input voltage and an output voltage in a band #1) of Patent Document 2. FIG. 7D corresponds to FIG. 10 (which is a graph explaining a DC offset generated by self-mixing) and FIG. 12 (which is a graph explaining a convergence time of a step response of the DC offset) in Patent Document 2. Explanation sentences added to FIGS. 7C and 7D are not present in Patent Document 2 and are added by the inventor of the present invention for explanation. The configuration and an operation overview of the receiver in FIG. 7 are as follows.
For respective frequency bands #1, #2, and #3, where frequency hopping is performed, capacitors #C1, #C2, and #C3 are respectively provided in series with series switches SW#1, SW#2, and SW#3 for path selection on baseband signal I/Q (In-Phase/Quadrature) lines in a receiver of a direct conversion receiving scheme. A resistance R is connected in shunt between the GND and an output of the capacitors #C1, #C2, and #C3, as a common output load. That is, a first-order RC HPF (High Pass Filter) circuit configuration is inserted into a baseband portion. The cutoff frequency of the HPF is set to a level capable of passing a subcarrier in the most vicinity of a DC component. Thus, the time constant of several symbols is needed.
According to the circuit in FIGS. 7A and 7B, capacitor switching is made in synchronization with the frequency hopping. Immediately before the frequency hopping from the frequency band #1 to other frequency band #2, the capacitor C#1 holds electric charge of a DC offset in the form of a step voltage. When the hopping is performed again to the frequency band #1, the circuit is operated to perform path connection so that a step voltage response is continued at the capacitor C#1. It is explained that, with this operation, a transition is made so that charging/discharging at the capacitor C#1 is to disappear after several hopping cycles, and an output of each RC circuit selected for each frequency hopping is stabilized at a steady state without DC offset.
Assume that removal of DC offset jump in the receiver is sought while fast frequency hopping is performed. Especially when fast frequency hopping is performed at a rather high speed as in the Multi Band OFDM UWB (Multi Band OFDM UWB) system, an approach to switching charging/discharging of the analog RC circuit by a switch is a sheer analog approach. Thus, rather close on/off path control is needed for the approach; A complete and stable cancelling effect assuming mass production in view of a malfunction at a time of path switching or variations in elements, operation, and temperature may not be able to be guaranteed.
In an OFDM receiver in the direct conversion system, an OFDM signal wave does not include a subcarrier signal with a center frequency, in general. That is, a DC component in a received baseband signal becomes Null (zero). Thus, it is possible to cut off DC offset amplification using the DC cut-off capacitance element Ccut.
Patent Document 2 is also based on a configuration in which the DC cut-off capacitance element Ccut is inserted. In this case, however, it is necessary to set the cut-off frequency of the first-order RC HPF using the DC cut-off capacitance element Ccut and the load R to be low so that the subcarrier in the most vicinity of the Null DC component is not suppressed. Consequently, the RC time constant would be increased to a period that extends over several symbols. For this reason, when the DC offset jump occurs, the DC offset jump for a long time of several OFDM symbols will be caused.
When the DC offset jump is made to converge by switching a path for the direct cutting capacitance element Ccut for the received baseband signal I/Q for each hopping and eliminating an electric charge transfer at the DC cut-off capacitance element Ccut for each hopping, convergence of the DC offset jump in an output of the DC cut-off capacitance element Ccut is determined by the RC time constant. It needs a period equivalent to several OFDM symbols. Thus, the DC offset jump cannot be immediately cancelled at the beginning of a received packet.
Patent Document 3 discloses a configuration in FIG. 8 (corresponding to FIG. 1 in Patent Document 3), in which a multi band OFDM_UWB transmitter/receiver is set to a Low-IF (intermediate frequency: intermediate frequency) to solve the problem in the transmitter/receiver in the direct conversion system. By performing rearrangement that rotates subcarriers after an FFT (Fast Fourier Transform) in the Low-IF receiver, the need for frequency conversion using a second local signal is eliminated. An AD conversion clock that is the same as in a direct conversion receiver is also used. On the other hand; detection at a preamble part to which the FFT is not applied can be performed by using a sequence obtained by multiplying an original preamble pattern by an IF frequency in advance.
Patent Document 4 discloses configurations in FIG. 9. FIG. 9A corresponds to FIG. 1 in Patent Document 4 (which is a block diagram of a transmitting device). FIG. 9B corresponds to FIG. 2 in Patent Document 4 (which is a block diagram of a receiving device). FIG. 9C corresponds to FIG. 3 of Patent Document 4 (which is a block diagram of a demodulation circuit). FIG. 9D corresponds to FIG. 12 in Patent Document 4 (which is a block diagram of a related art receiving device). Patent Document 4 explains a configuration of a correlation detector on a receiver side in the case of a modulation/demodulation system that uses a fast frequency hopping (Fast Frequency Hopping) method for frequency spreading and correlation detection. The fast frequency hopping method is a type of (CDMA (Code Division Multiple Access)/SS transmission method. The correlation detector uses a wireless transmission system in which 1/0 symbols are expressed as frequency hopping patterns of a combination of a plurality of frequencies. The correlation detection on the receiver side has been hitherto performed by non-coherent detection (scheme in which K different transmission hopping carrier frequencies have no phase continuity before and after hopping and phase synchronization is not achieved on a receiver side as well, which means that LO frequencies on the receiver side are not phase synchronized with the transmission carrier frequencies). Patent Document 4 proposes the frequency hopping system in which correlation detection on the receiver side is performed by coherent detection while securing phase continuity among hopping carriers on a transmitting side, thereby aiming at improvement of reception sensitivity performance by 6 dB (which is equivalent to improving a required C/N ratio by 6 dB) theoretically.
The configuration of this receiver adopts a single conversion method. A first LO frequency on the receiver side is hopped with K frequency patterns which are the same as in transmission, for supply to a synthesizer (MIX), in the form of frequencies f1+fIF, f2+fIF, . . . fK+fIF. Each of received signals down-converted to an IF band assumes the same IF frequency when hopping synchronization between the transmitter and receiver sides has been completely obtained. This signal resulting from this conversion undergoes complex (I/Q) envelope detection by a quasi-synchronization detector, and is the divided into K complex envelopes by a time-series switch. Then, the K complex envelopes are respectively multiplied by K complex coefficients. Then, using a minimum square method, one complex envelope is computed from synthesis using the K complex envelopes, and symbol determination is made by a determination circuit. In that case, by automatically controlling the K complex coefficients so that an input/output error of the determination circuit is eliminated, an optimal reception status is adaptively maintained. That is, this operation is equivalent to coherent detection continuously involving phase synchronization.
It is described that this receiver has a configuration having LOs (local oscillators) of which respective hopping frequencies are fixed and MIXs (mixers) in order to extract the frequency hopping pattern of an incoming hopped received wave. This receiver is not based on the direct conversion scheme. A synthesized output (mixer output) has an IF frequency, and this receiver is based on the single conversion scheme.
It has been typically desired that, when frequency hopping is performed in the Multi Band OFDM UWB system, an LO unit complete the frequency hopping within a zero segment (of a total of 37 clocks equivalent to 70.1 nsec) that is present in each hopping symbol (of an effective FFT (Fast Fourier Transform) length of 128 clocks equivalent to 242.4 nsec), and further within a guard interval segment (of 5 clocks of 9.5 nsec) sandwiched between two types of zero padding segments (of a total of 32 clocks equivalent to 60.6 nsec) in the zero segment. It is highly difficult to perform LO frequency switching within such a short period. Generally, it has been considered to be difficult to perform frequency switching within a period of 9.5 nsec due to the influence of the stray capacitance of a high-speed frequency switching switch and a transmission path even if the switch is provided. In order to solve this problem, Patent Document 5 provides a Multi Band OFDM UWB wireless receiver device capable of implementing frequency hopping without using the high-speed frequency switching switch. FIGS. 10A, 10B, 10C, 10D, 10D and 10F are respectively reproductions of FIGS. 1, 2, 3, 4, 5, and 6 in Patent Document 5.
As a basic receiver configuration, three sub-hands are shared by a switch, a BPF, and an LNA before QDEMs for direct conversion. The switch, BPF, and LNA are implemented as one system. Three systems corresponding to the number of frequency hoppings are implemented in chains from an analog baseband portion after the QDEMs to AD converters. Then, a LO frequency is fixed for each hopping band in advance, and the receiver is set to a ready state for reception. In this configuration, frequency hopping is completely eliminated. As the other configuration, a configuration where frequency hopping is sequentially performed is used. In the other configuration, blocks are implemented after the QDEMs in two systems (in FIGS. 10C, 10D, 10F: FIGS. 3, 4, and 6 in Patent Document 5), and a symbol segment immediately before a next symbol is used to cope with another remaining Band hopping, and frequency switching for an unused QDEM/LO system on one side is performed with a sufficient time allowance taken for a next hopping frequency, thereby performing hopping on the receiver side one after another. Further, switches are respectively provided after I/Q baseband AD converters (three pairs of the I/Q baseband AD converters in first and three exemplary embodiments in Patent Document 5 and two pairs of the I/Q baseband AD converters in second and fourth exemplary embodiments in Patent Document 5) each provided for each receiver chain after each QDEM in each exemplary embodiment. Each of two series of I/Q symbol sequences output from the respective AD converters as a result of each hopping (each symbol sequence being composed of 165 or 192 quantized elements quantized by each AD converter) is temporarily stored in one or more shift registers (one shift register being implemented for each I/Q signal in the first and second exemplary embodiments in Patent Document 5 and three shift registers being implemented for each I/Q signal in the third and fourth exemplary embodiments in Patent Document 5) before an FFT. Then, a cyclic prefix process is applied in order to cope with a delay wave caused by multi-path fading. Then, the FFT is applied one after another in the order of hopping symbols.
A secondary effect other than provision of fast hopping replacement means in Patent Document 5 is as follows. When a delay spread caused by multi-path fading generated according to various propagation environments becomes equal to or larger than an interframe space (of 70.1 nsec) between adjacent effective symbols, which is a sum of the zero padding segment and the guard interval segment, interference between the symbols occurs. Degradation in manifest reception characteristics is thereby caused.
in the configuration in Patent Document 5, quantized elements of an I/Q symbol sequence corresponding to each hopping frequency after AD conversion (sampling elements quantized by each AD converter) are temporarily stored in the one or more shift registers provided in a digital baseband space before the FFT. Then, the cyclic prefix process for recovering a loss caused by a delay can be slowly performed at discontinuous symbol intervals for the each hopping frequency. Thus, the third and fourth exemplary embodiments in Patent Document 5 have an effect that fading compensation for a multi-path delay wave (for which a maximum allowable delay being 64 clocks/121.2 nsec) longer than an allowable value that can be compensated for in most instances (where the zero padding segment is a total of 32 clocks/60.6 nsec+the guard interval is 5 clocks/9.5 nsec, totaling 37 clocks/70.1 nsec) can be performed using the cyclic prefix process.
When the third and fourth exemplary embodiments in Patent Document 5 are improved and the FFT can be applied for each shift register, there is also a possibility that at a time of 3 Band hopping, cyclic prefix compensation can be performed within a frame of an effective FFT length of 242.4 nsec after a maximum 695.1 nsec multi-path delay in the improvement of the third exemplary embodiment in Patent Document 5 and a maximum 382.6 nsec multi-path delay in the improvement of the fourth exemplary embodiment in Patent Document 5 have been once stored.    [Patent Document 1]
JP Patent Kokai Publication No. JP-P-2005-210170A    [Patent Document 2]
JP Patent Kokai Publication No. JP-P-2006-203686A    [Patent Document 3]
JP Patent Kokai Publication No. JP-P-2006-121546A    [Patent Document 4]
JP Patent No. 2700746    [Patent Document 5]
JP Patent Kokai Publication No. JP-P-2006-20072A